1. Field of the Invention
The present invention relates to reception systems for radar and for radio communications in which interference cancellation takes place in a correlation loop. More particularly, the invention relates to means for nulling the phase shift in such correlation cancellation loops.
2. Description of the Prior Art
On radar systems, as well as in many communications systems, a highly directional main antenna is provided for receiving information. All such directional antennas have minor lobes to either side of the main lobe. The interference received via the side lobes may interfere with the desired signal impinging on the main lobe. Devices which attack this problem have been termed "side lobe cancellers". Known "side lobe cancellers" have taken the form of adaptive signal processors in which interference is derived in less directional antennae separate from the directional main antenna. The interference is then reconstructed by correlation techniques in such a way that when subtracted from the main return, the interference is cancelled.
The interference, while dealt with in general terms, may take the form of a jamming signal in a radar system; or, in a communications system, a signal from a nearby source. Such side lobe cancellers have in common, a cancellation summer in which interference present in the main antenna return is cancelled with that in the auxiliary antenna. Prior to the cancellation, however, correlation is undertaken between the interference in the auxiliary antenna and that in the main antenna return, resulting ideally in a complex low frequency or d.c. quantity, the "correlation" representing the relative magnitude and phase relationships of the two signals. This "correlation" can then be used as a complex "weight", which when multiplied by the interference in the auxiliary signal, will cause a more nearly ideal cancellation.
A common requirement in all such systems is that the feedback loop must operate with high stability. A well known requirement in such loops is that the phase shift around the loop be 180.degree..
Accordingly, it has been proposed to inject a signal into the main and auxiliary channel to "activate" the side lobe canceller loop, and then interrupt the loop to sense a 180.degree. phase shift, while the phase shift around the loop is being manually adjusted. In general, the manual adjustment is complicated by the need for relatively highly skilled personnel to effect the adjustment, and is a relatively complex procedure.